I saw someone asking this question.
Is there a German word meaning 'a spontaneous, unscheduled, unexpected, abrupt meeting' starting with 'Knall~' ?
There is a word Knalleffekt used for a spontaneous, unexpected, sudden event (of any kind). But I am not aware of a word starting with Knall (which means bang) especially for a meeting.
You could of course create one, e.g. Knalltreffen, but then you are in the realm of poetry, not everyday or business communication. (And pay attention to what Takkat adds regarding Knalltreffen in his separate answer!)
The Duden Universalwörterbuch (some current edition) lists for Knall:
as well as the adjectives
None of them means "sudden meeting".
However, you can think of various words starting with Blitz (lightning). The following terms are hardly to be found in contemporaneous dictionaries, but Blitz is more productive than Knall, meaning that you can create new words without being looked at with puzzlement. So Blitzkonferenz would pass almost unrecognized as a neologism, also Blitz-Meeting, Blitzbesprechung, Blitzgespräch, Blitztreffen. I could imagine finding these words used in some (informal) business communication, e.g. in e-mails to schedule a quick meeting.
This needs to be mentioned:
The only context I heard of Knalltreffen is from dingy contact queries on certain red light district announcements.
Examples (links for obvious reasons not included but with Google you will find them easily if the source is not banned on your machine):
- Suche einen aufgeschlossenen Charmeur für geile Knalltreffen.
- Suche für unregelmäßige Knalltreffen gesunde, schlanke Herren.
In these cases Knalltreffen is used for the original intention behind such contacts where knallen is used in its vulgar meaning for having sexual intercourse. If any of these meetings follow they may be spontaneous but not really unscheduled, or unexpected. I can not exclude that the meaning may be regionally different, but where I live it is definitely not a word people know.
I'm not aware of a substantive, associating sudden occurence with Knall. I also find this plausible, since Knall more often than not also indicates the termination of something (like a bursting balloon).
So Blitz- as in Blitzkrieg is the more likely choice (and its also used in English if I consider flash mob).
There is also the term ad hoc leaned from Latin (Duden gives the meaning aus dem Augenblick heraus, probably translated to instantaneous) if just the improvised nature shall be pointed out, so the usual scheduling activities with notification weeks before are bypassed.